Inner-wall modification of a commercial polymeric microfluidic chip using pulsed He/O2 and Ar/O2 µplasmas

Abstract

Pulsed He/O2 and Ar/O2 µplasmas were used to modify the postbonded microchannels of a commercial cyclic olefin copolymer microfluidic chip. The µplasmas were generated throughout a 60-mm-long microchannel with a cross section of 340 × 86 µm2 at atmospheric pressure using Pt wire electrodes under discharge conditions of spark voltages of 6–8 kV and a pulse repetition frequency of 50 Hz with a duty ratio of 0.2%. From optical emission spectroscopy results, the electronic excitation temperature was estimated to be approximately 2.1 × 104 K for both He and Ar µplasmas, and atomic oxygen radicals were increasingly produced with increasing O2 gas concentration. The plasma-treated microchannel inner wall was analyzed by X-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy. The wettability of the microchannel inner walls treated with both He/O2 and Ar/O2 shows the following tendencies; the wettability increased by about 4 times that of a raw microchannel at O2 gas concentrations of 5–10% and then the wettability decreased with increasing O2 gas concentration over 10%. It is found that the wetting property of the microchannels depends on both the oxygen polar groups, such as –C–O–, C=O, and COOH, introduced onto the inner walls and the surface roughness due to plasma etching.